Following trauma exposure a substantial number of individuals will go on to develop psychopathology, such as posttraumatic stress disorder (PTSD), and many also develop co-occurring substance use, such as marijuana use. Their co-occurrence is costly to both the victim and society as a whole. The co-occurrence of PTSD and marijuana use is predictive of poorer outcome and increased drop out from PTSD treatment. Exposure therapy, a first line intervention for PTSD, is theorized to work through extinction processes, and marijuana has known effects on extinction learning. Thus, a better understanding of the relationship between marijuana and extinction learning, in both experimental and treatment contexts, can help us better understand how to best intervene with this population. This project is designed to integrate basic and clinical science to translate mechanisms of recovery from PTSD to testing an intervention in individuals with PTSD and marijuana use. We will recruit 72 individuals with PTSD and varying levels of marijuana use, 36 with current heavy marijuana use and 36 without current marijuana use, to complete a conditional discrimination and extinction task in order to understand the relationship between marijuana and fear learning in individuals with pathological fear. Following this task, participant will be randomized to either a brief 6 session imaginal exposure protocol (IE) or a standard 10 session prolonged exposure protocol (PE) to explore whether the adapted short form treatment shows increased feasibility (efficacy, reduced drop out) for individuals with co-occurring PTSD and marijuana use. We will also be able to test the link between basic fear and recovery processes by looking at the extinction task as a predictor of treatment response for individuals with and without marijuana use. This research is significant in its potential to identify a predictr of treatment response, to test an underlying mechanism of recovery for PTSD in individuals with PTSD and co-occurring marijuana use, and to test feasibility of a novel intervention for a difficul to treat population.